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Abstract:

Numerous gas field developments worldwide have maximised profitability by
utilising high productivity, large-bore horizontal completions. However, this
production strategy is associated with significant risk in reservoirs with a large
component of water-drive, particularly if the gas column is thin, the structural relief is
small, and the reservoir is producing from only one or two wells. Rapid water influx
('cresting') can lead to early water breakthrough which effectively 'kills' the well.
Predicting the time to breakthrough requires proper consideration of the interaction
between reservoir properties and production strategy.
We present an integrated geoscience and engineering study to quantify the impact of
depositional heterogeneity on gas-water fluid flow behaviour when horizontal wells
are produced at high rates in the presence of a bottom water aquifer. Gas production
is simulated from a reservoir model of a single shoreface-shelf parasequence, that is
conditioned to' high-resolution outcrop data. Novel surface-based modelling
techniques ensure that cr\rical heterogeneities are captured without recourse to
upscaling. The model is representative of gas reservoirs in the Columbus Basin,
offshore Trinidad and Tobago, which are currently being developed using a small
number of high rate horizontal wells.
We find that an understanding of well location with respect to the spatial distribution
of non-reservoir. units is critical to managing production rate and delaying water
break1hrough. This is because enhanced recovery occurs when heterogeneity is
suppressing cresting, rather than because production is 'outrunning' the aquifer.
Furthermore, when the well is protected from water cresting, aquifer support is
actually observed to improve ultimate recovery. Simulation models should possess
sufficient geological detail to describe the location and 3D architecture of baffles to
flow. Other aspects of the reservoir description, such as permeability contrasts
between facies, are much less significant. Our results explain how material balance
approaches can be interpreted to improve predictions of production performance when
there is a significant risk of water cresting, or the aquifer response is modified by
depositional heterogeneity.